This invention relates to methods and apparatus for signal processing, and more particularly to such methods and apparatus for mitigating the effects of signal fades, temporary blockages or severe channel impairments in an in-band on-channel digital audio broadcasting system.
Digital Audio Broadcasting (DAB) is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM or FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested.
FM IBOC DAB broadcasting systems have been the subject of several United States patents including U.S. Pat. Nos. 5,465,396; 5,315,583; 5,278,844 and 5,278,826. More recently, a proposed FM IBOC DAB signal combines an analog modulated carrier with a plurality of orthogonal frequency division multiplexed (OFDM) sub-carriers placed in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier.
One AM IBOC DAB approach, set forth in U.S. Pat. No. 5,588,022, presents a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. Using this approach, an amplitude-modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude-modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each digitally modulated carrier signal is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally-modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
Radio signals are subject to intermittent fades or blockages that must be addressed in broadcasting systems. Conventionally, FM radios mitigate the effects of fades or partial blockages by transitioning from full stereophonic audio to monophonic audio. Some degree of mitigation is achieved because the stereo information which is modulated on a sub-carrier, requires a higher signal-to-noise ratio to demodulate to a given quality level than does the monophonic information which is at the base band. However, there are some blockages which sufficiently xe2x80x9ctake outxe2x80x9d the base band and thereby produce a gap in the reception of the audio signal. IBOC DAB systems should be designed to mitigate even those latter type outages in conventional analog broadcast, at least where such outages are of an intermittent variety and do not last for more than a few seconds. To accomplish that mitigation, digital audio broadcasting systems may employ the transmission of a primary broadcast signal along with a redundant signal, the redundant signal being delayed by a predetermined amount of time, on the order of several seconds, with respect to the primary broadcast signal. A corresponding delay is incorporated in the receiver for delaying the received primary broadcast signal. A receiver can detect degradation in the primary broadcast channel that represents a fade or blockage in the RF signal, before such is perceived by the listener. In response to such detection, the delayed redundant signal can be temporarily substituted for the corrupted primary audio signal, acting as a xe2x80x9cgap fillerxe2x80x9d when the primary signal is corrupted or unavailable. This provides a blend function for smoothly transitioning from the primary audio signal to the delayed redundant signal.
The concept of blending from a DAB signal of an IBOC system to an analog, time delayed audio signal (AM or FM signal) is described in U.S. Pat. No. 6,178,317. The implementation implied in that patent assumed that the analog signal can be delayed in real time through brute force hardware processing of the signal in real time where relative delays can be controlled precisely. However, it would be desirable to construct a delay control that can be implemented using non-real-time programmable digital signal processors (DSP). This invention provides a DAB signal processing method including diversity delay and blend functions that can be implemented using programmable DSP chips operating in non-real-time.
This invention provides a method for transmitting a composite digital audio broadcast signal having an analog portion and a digital portion to mitigate intermittent interruptions in the reception of the digital audio broadcast signal. The method comprises the steps of arranging symbols representative of the digital portion of the digital audio broadcast signal into a plurality of audio frames, producing a plurality of modem frames, each of the modem frames including a group of the audio frames, and adding a frame synchronization signal to each of the modem frames. The modem frames are then transmitted along with the analog portion of the digital audio broadcast signal, with the analog portion being delayed by a time delay corresponding to an integral number of the modem frames. The invention also encompasses radio receivers and transmitters which process signals according to the above methods.